The objective of this research is to investigate renal functional adaptation to compensatory growth and to demonstrate a transplacental humoral factor of maternal origin that mediates enhanced fetal renal growth in utero. Compensatory and/or enhanced renal growth will be measured by determining the rate of 14C-choline incorporation into renal phospholipids in cell membranes in cortical kidney slices. Functional adaptation will be measured by peformance of standard clearances. In vivo studies will be done in ewes pregnant with twins. After cesarean section for removal of one twin as the control, a maternal left or bilateral nephrectomy will be done. Four hours later, the remaining twin will be delivered. The mean results of kidney slices from the second twin will be compared to those for the first twin as well as to normal controls and "sham" operated ewes. Compensatory growth in the maternal renoprival right kidney will serve as a positive control for elaboration, by the ewe, of a humoral renal growth-promoting substance after maternal left nephrectomy. In vitro studies will be done by removing blood from the ewes studied above. The serum will be immediately separated and frozen at -20 degrees C for 72 hours. Cortical kidney slices from fetal lambs will be incubated in media containing 14C-choline and maternal sera. Additional cortical kidney slices from adult rats and liver slices from fetal lambs will be used to study species ad tissue specificity, respectively, of the humoral factor. Positive results after maternal bilateral nephrectomy will be highly suggestive of an additional extrarenal origin of the humoral factor.